The roof is probably not trussed, they existed but were not very common in Cdn residential construction until the 70's. Any GOOD contractor can give you a quote because he has likely done the job before, will be able to look and see if the load points at each end of a beam will fall on existing load points that are sufficient to transfer the load to the basement. He likely knows what beam is required and can tell you if the beam could be reccesed into the ceiling and metal connectors used to transfer ceiling joists.
He can probably suggest and architect or engineer that he already has relationships with. In the end you will need drawings but a contactor will know what they will cost + - .
possibly he also knows which architect will want to build a spiral staircase into the attic because its art...... when all you need is a load calculation.
trip to your local municipal office is always a good idea , they aren't supposed to recommend a contractor but if your very polite they will steer you away from someone you dont want to hire.
other caveat ; you need no license or training to be a general contractor , thats why there are so many of them, they "contract" out to licensed professionals. Not all of them should be in business.
